A new Yale-led study quantifies for the first time the primary causes of the "urban heat island" (UHI) effect, a common phenomenon that makes the world's urban areas significantly warmer than surrounding countryside and may increase health risks for city residents.

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In an analysis of 65 cities across North America, researchers found that variation in how efficiently urban areas release heat back into the lower atmosphere — through the process of convection — is the dominant factor in the daytime UHI effect. This finding challenges a long-held belief that the phenomenon is driven principally by diminished evaporative cooling through the loss of vegetation.

The effects of impaired "convective efficiency" are particularly acute in wet climates, the researchers say. In cities such as Atlanta, Georgia, and Nashville, Tennessee, this factor alone contributes a 3-degree C rise in average daytime temperatures, according to the study, published July 10 in the journal Nature.

The phenomenon could have profound impacts on human health in cities worldwide as mean global temperatures continue to rise — and as more and more people move into cities — said Xuhui Lee, the Sara Shallenberger Brown Professor of Meteorology at the Yale School of Forestry & Environmental Studies (F&ES) and one of the study's authors.

"There is a synergistic relationship between climate conditions and the urban heat island," Lee said. "This relationship suggests that the urban heat island will exacerbate heat wave stress on human health in wet climates where temperature effects are already compounded by high humidity.

"This is a huge concern from a public health perspective."

For years scientists have recognized the primary causes of the UHI effect. In addition to the changes in convection efficiency and evaporative cooling, these include the tendency of buildings, pavement, and other structures to store more heat than vegetation and soil; heat generated by human-built industrial systems; and changes to the albedo of the Earth's surface. (Albedo refers to the proportion of sunlight or radiation reflected by the surface of the planet. Light-colored parking lots, for instance, reflect more sunlight back into space than darker surfaces.)